SwiftUI - passing managed object context to model

Question

Passing a managed object context to a view in SwiftUI is easy enough using an @Environment variable. But getting that same context to the model and view model, not so much. Here is what I tried:

I created a property called context in the view model. In the view, I pass the managed object context lazily. The problem is that now I'm getting an error when I call a view model's method from the view - "Cannot use mutating getter on immutable value: 'self' is immutable". The method worked before I added the context, so why did it stop working? And, more importantly, how do I make it work?!

Model:

struct Model {

   //use fetch request to get users

   func checkLogin(username: String, password: String) {
      for user in users { 
        if username == user.userEmail && password == user.password {
            return true
        }
    }
    return false
   }
}

View Model:

class ViewModel {
   var context: NSManagedObjectContext
   private var model = Model()

   init(context: NSManagedObjectContext) {
      self.context = context 
   }

   func checkLogin(username: String, password: String) -> Bool {
      model.checklogin(username: username, password: password)
   }
}

And finally, the View:

struct LoginView: View {
   @Environment(\.managedObjectContext) var moc
   lazy var viewModel = ViewModel(context: moc) 

    //Login form
    
    Button(action: {
       if self.viewModel.checkLogin(username: self.email, password: self.password) { 
       //ERROR: Cannot use mutating getter on immutable value: 'self' is immutable
          //allow login
       }
    }) {
        Text("login")
    }
}

Answer 1

You cannot use lazy var in View, because it is immutable. Here is possible approach to solve your case

class ViewModel {
   var context: NSManagedObjectContext?
   private var model = Model()

   init(context: NSManagedObjectContext? = nil) {
      self.context = context
   }

   func checkLogin(username: String, password: String) -> Bool {
      return model.checkLogin(username: username, password: password)
   }
}

struct LoginView: View {
   @Environment(\.managedObjectContext) var moc

   private let viewModel = ViewModel()   // no context yet here, so just default

    //Login form
    var body: some View {
        Button(action: {
           if self.viewModel.checkLogin(username: self.email, password: self.password) {
              //allow login
           }
        }) {
            Text("login")
        }
        .onAppear {
            self.viewModel.context = moc  // << set up context here
        }

    }
}